The present invention relates to a phase change type optical recording medium.
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for rewriting. One typical rewritable optical recording medium is of the phase change type wherein the recording layer is irradiated with laser beam to change its crystallographic state whereupon a change of reflectance by the crystallographic change is detected for reading. Optical recording media of the phase change type can be overwritten by modulating the intensity of a single light beam, i.e., by modulating recording light in such a way that it has a plurality of power levels comprising a recording power level and an easing power level. In addition, the optical recording medium of the phase change type is simpler in terms of the optical system of the driving mechanism as compared with magneto-optical recording media.
Most optical recording media of the phase change type use chalcogenides such as Ge-Te and Ge-Sb-Te based materials which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state. In recent years, it has also been proposed to use compounds known as chalcopyrites. Chalcopyrite compounds are extensively investigated as compound semiconductor materials, and applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib-IIIb-VIb2 or IIb-IVb-Vb2 as expressed in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily identified by X-ray structural analysis and their basic characteristics are described in Physics, Vol. 8, No. 8 (1987), page 441, Denki Kagaku (Electrochemistry), Vol. 56, No. 4 (1988), page 228, and other literature. Among the chalcopyrite compounds, AgInTe2 is known to be applicable as a recording material by diluting it with Sb or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s. See JP-A""s 3-240590, 3-99884, 3-82593, 3-73384 and 4-151286. In addition to these phase change type optical recording media using chalcopyrite compounds, JP-A""s 4-267192, 4-232779 and 6-166268 disclose a phase change type optical recording media wherein an AgSbTe2 phase forms upon the crystallization of a recording layer.
In the phase change type optical recording medium, the substrate is provided with tracking grooves (guide grooves). In some cases, these grooves may carry address information thereon. So far, it has been common to form recording marks within the grooves or a region between adjacent grooves (land). In recent years, however, a land-groove recording system wherein both grooves and lands are used in the form of recording tracks has been proposed due to the need of achieving high-density recording by making the pitch of recording tracks narrow (JP-B 63-57859). In the present disclosure, it is noted that the term xe2x80x9cgroovexe2x80x9d refers to a relatively concave area of a substrate having a recording layer formed thereon to allow recording and reading light to be incident on the recording layer through the substrate, i.e., an area located nearer to the side of the substrate on which light is incident, and the term xe2x80x9clandxe2x80x9d refers to a relatively convex area of the substrate, i.e., an area located farther off the side of the substrate on which light is incident.
However, the use of the land-groove recording system leads to a problem that it is difficult to obtain equivalent properties at the lands and grooves.
To solve this problem, for instance, JP-A 7-134838 proposes that while the groove has a specific depth at substantially the same groove and land width, the relation between the thickness of the second dielectric layer formed on the recording layer and the reflective layer formed on the second dielectric layer is specified, thereby making uniform the recording properties at the land and groove. However, this publication gives no example of the recording layer comprising an alloy other than the Ge-Sb-Te alloy.
A recording layer having an Ag-In-Sb-Te based composition containing Ag, In, Sb and Te as main components is characterized by being higher in sensitivity and clearer in the edges of recording marks than that based on a Ge-Sb-Te composition used in the example of the aforesaid JP-A 7-134838. Thus, this recording layer is suitable for mark edge recording. Mark edge recording enables information to be carried on both ends of recording edges, and so makes higher-density recording possible as compared with mark position recording where information is carried on the position of recording marks, resulting in recording capacity improvements.
As a result of examination of the land-groove recording properties of an optical recording medium using the aforesaid Ag-In-Sb-Te based recording layer, the inventors have now found that jitter and repetitive overwriting properties at the land are still unsatisfactory.
An object of the present invention is to provide a phase change type optical recording medium comprising a recording layer having an Ag-In-Sb-Te based composition and used in a land-groove recording mode, which has reduced land jitter while increased land jitter in association with repetition of overwriting is reduced.
Such an object is achieved by the inventions defined below as (1) to (3).
(1) An optical recording medium comprising a phase change type recording layer on a surface of a substrate with reading light incident on the recording layer through the substrate, and a dielectric layer provided in contact with the recording layer and on a side of the recording layer on which the reading light is incident, said substrate having a groove acting as a guide groove and a land between adjacent grooves so that the groove and land are used as recording tracks, wherein:
said phase change type recording layer contains Ag, In, Sb and Te as main components,
said phase change type recording layer has a thickness tR defined by tRxe2x89xa6xcexp/20 where xcexp is a wavelength of said reading light, and
said dielectric layer has a thickness td1 that is represented in terms of an optical path length at said wavelength xcexp and defined by xcexp/2xe2x89xa6td1.
(2) The optical recording medium of (1) above, wherein xcexp/60xe2x89xa6tR.
(3) The optical recording medium of (1) above, which further comprises a reflective layer formed of a metal or a semi-metal, with said recording layer interleaved between said reflective layer and said dielectric layer.